[PDF] Chapter 1 Organic Compounds: Alkanes

[PDF] Chapter 1 Organic Compounds: Alkanes - Angelo State University

Learn how to recognize the alkane functional group in organic compounds Butane and isobutane are structural isomers of each other Butane Isobutane

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Since then, it has been recognized that organic molecules carbon atoms means that the base name of this compound will be butane

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of organic molecules and how structural formulas written in two tures such as butane, C,H,,; 1-butene, C4H8; or 1,3-butadiene, C,H,, but it is

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There are many types of organic molecules Hydrocarbons are an example of some of the more simple organic compounds since they only contain carbon

[PDF] 1 Organic Compounds are most likely to

7 Which of the following pairs represent structural isomers? propane and butane 2-methylbutane and butane 2-methylpropane and propane

[PDF] Chapter 1 Organic Compounds: Alkanes - Angelo State University

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Chapter 1 Alkanes

Mr. Kevin A. Boudreaux

Angelo State University

CHEM 2353 Fundamentals of Organic Chemistry

Organic and Biochemistry for Today (Seager & Slabaugh) www.angelo.edu/faculty/kboudrea

Chapter Objectives:

•Learn the differences between organic and inorganic compounds. •Learn how to identify isomers of organic compounds. •Learn how to write condensed, expanded, and line structures for organic compounds. •Learn how to recognize the alkane functional group in organic compounds. •Learn the IUPAC system for naming alkanes and cycloalkanes. •Learn the important physical and chemical properties of the alkanes.

Chapter 1 Organic Compounds:

Alkanes

Organic chemistry nowadaysalmost drives me mad. Tome it appears like a primevaltropical forest full of themost remarkable things, adreadful endless jungle intowhich one does not dareenter, for there seems tobe no way out.

Friedrich Wöhler

Chapter 1 Alkanes

2 3 4

What Do We Mean By "Organic"?

• In everyday usage, the word organiccan be found in several different contexts:

- chemicals extracted from plants and animals were originally called "organic" because they came from living organisms.

- organic fertilizers are obtained from living organisms. - organic foods are foods grown without the use of pesticides or synthetic fertilizers.

• In chemistry, the words "organic" and "organic chemistry" are defined a little more precisely:

Chapter 1 Alkanes

3 5

What is Organic Chemistry?

•Organic chemistryis concerned with the study of the structure and properties of compounds containing carbon.

- All organic compounds contain carbon atoms. - Inorganic compounds contain no carbons. Most inorganic compounds are ionic compounds.

• Some carbon compounds are not considered to be organic (mostly for historical reasons), such as CO, CO

2 , diamond, graphite, and salts of carbon-containing polyatomic ions (e.g., CO 32-
, CN - ). •Inorganic chemistry is the study of the other elements and non-carbon containing compounds. 6

The Periodic Table

• There are 92 naturally occurring elements, and many artificial ones, in the (in)famous Periodic Table:

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

CePrNdPmSmEuGdTbDyHoErTmYbLu

ThPaUNpPuAmCmBkCfEsFmMdNoLr

NaMgAlSiPSClAr

LiBeBCNOFNe

HHe

FrRaAcRfDbSgBhHsMtDsRgCn

I A II A

III B IV B V B VI B VII B

III BI B II BIII A IV A V A VI A VII A

VIII A

1 2 3 4 5 6 7

Lanthanides

Actinides

FlLv

Chapter 1 Alkanes

4 7

The Periodic Table of Organic Chemistry

• Organic chemists look at the Periodic Table a little differently:

CrMnFe CoNiCuBr

Pd I PtMg

AlPSClB

NOFH C 8

Origins of Organic Chemistry

• Organic literally means "derived from living organisms" - organic chemistry was originally the study of compounds extracted from living organisms and their natural products.

• It was believed that only living organisms possessed the "vital force" necessary to create organic compounds ("vitalism").

• This concept started to change in 1828 after Friedrich Wöhler showed that it was possible to make urea, a known "organic compound" from a mineral source:

NH 4+ -

OCNHeat

Ammonium

Cyanate UreaC

O NN HH HH

Chapter 1 Alkanes

5 9

Origins of Organic Chemistry

• What this and later experiments showed was that "organic" molecules - even those made by living organisms - can be handled and synthesized just like minerals and metals

• What was special about these molecules was that they contained the element carbon. 10

What's So Great About Carbon?

• Carbons atoms can be linked by strong, stable covalent bonds. C neutral carbon, C C carbon cation, C 4+ C carbide anion, C 4- CHH HH CH H HH

Chapter 1 Alkanes

6 11

What's So Great About Carbon?

• Carbon atoms can form stable bonds to many other elements(H, F, Cl, Br, I, O, N, S, P, etc.). Most organic compounds contain a few hydrogens, and sometimes oxygen, nitrogen, sulfur, phosphorus, etc.

• Carbon atoms can form complex structures, such as long chains, branched chains, rings, chiralcompounds (having a particular "handedness"), complex 3D shapes, etc.

• Because of this variety in bonding and complexity, carbon atoms can form a tremendous variety of compounds. More than 16,000,000 organic compounds are known, as opposed to about 600,000 inorganic compounds.

What's So Great About Carbon?

• Complex organic compounds can perform a number of useful biological functions(vitamins, carbohydrates, lipids, proteins, enzymes, ATP, DNA, RNA are all organic compounds) which are studied in biochemistry.

• Complex organic compounds are present in the foodswe eat (carbohydrates, proteins, fats, etc.)

•Most medicines, whether they come from a chemical plant or a green plant, are organic compounds.

•Most fuelsare organic compounds (wood, coal, natural gas, gasoline, kerosene, diesel fuel, oil, and other petroleum-based products).

• Complex organic compounds are also useful in technology (paints, plastics, rubber, textiles, etc.).

Chapter 1 Alkanes

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Organic vs. Inorganic Compounds

• Organic compounds are held together by covalent bonds, while inorganic compounds are held together by ionic bonds.

CH H

HHmethane

sodium chloride Na + Cl - Na + Cl - Na + Cl - CH H HH Na + Cl - Na + Na + Cl - CH H HH Cl - Na + Cl - Cl - Na + 14

Organic vs. Inorganic Compounds

Property Organic Inorganic

Bonding within moleculesCovalent Often ionic

Forces between moleculesGenerally weak Quite strong

Normal physical state

Gases, liquids, or low melting-point solidsUsually high melting-point solids

Flammability Often flammable

Usually nonflammable

Solubility in water Often low Often high

Conductivity of aqueous solutionsNonconductor ConductorTable 1.1Properties of typical organicand inorganic compounds.

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Atomic Orbitals on Carbon

• A carbon atom does not form ions easily, since it has four valence electrons (1s 2 2s 2 2p 2 ). It satisfies the octet rule in compounds by sharing electrons. • These are the orbitals that exist on atomic carbon (not connected to anything). sorbital porbital 2s2p

Energy

Chapter 1 Alkanes

9 17

Hybrid Orbitals

• When carbon atoms form bonds with each other, we describe the resulting bonds using hybrid orbitals, which are formed by mixing (hybridizing) the carbon's atomic orbitals. (Linus Pauling, 1950s)

• When carbon atoms bond to 4 other atoms, the 2sorbital and all three 2porbitals in the valence shell combine to produce four sp

3 hybrid orbitals: ++++++ 2s

1 atomic

orbital 2p

3 atomic

orbitals sp 3

4 hybrid

orbitals 18 2s2p

Energy

1ssp 3 1s hybridization

Hybrid Orbitals

• All four sp 3 orbitals are at the same energy level, with one electron in each hybrid orbital.

Chapter 1 Alkanes

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The Shape of an sp

Carbon

• In order to get as far away from each other as possible (thus minimizing electron-electron repulsions), the sp

orbitals are arranged in the shape of a tetrahedronaround the central carbon atom, with bond angles of 109.5º.

CC109.5°

sp 3 sp 3 sp 3 sp 3 20

The Shape of an sp

Carbon

Chapter 1 Alkanes

11 21

Bonding in Ethane

• Bonds arise from the overlapof orbitals on adjacent atoms. - End-on-end overlap of sp 3 orbitals produces a -bond(sigma bond). - All single bonds are -bonds. -Free rotationis possible around -bonds. • Each carbon in the ethane molecule, CH 3 CH 3 , is sp 3

-hybridized and tetrahedral in shape. Free rotation is possible around the C - C bond. (See next slide)

Bonding in Ethane (CH

3 CH 3 ) CHCH H H H H C HC H H H H H

Chapter 1 Alkanes

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Carbon Chains

• Each carbon atom can form four bonds, either to other carbon atoms, or to different atoms (such as H, O, N, S, P, etc.)

=CC

Threemoresites

to make bonds CCCC

CCCCCCCCC

CCCCCC

etc. 24

Multiple Bonds

• Carbon atoms form four bonds to other things, but sometimes those bonds are multiple bonds(double or triple bonds):

CCCC triplebondresults from the sharingof sixelectrons

CCCCCCCC

double bondresults from the sharingof fourelectronssingle bond results from the sharing of twoelectrons

Chapter 1 Alkanes

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Isomers

•Isomers - compounds having identical molecular formulas, but different arrangements of atoms. •Structural Isomers - the atoms in each molecule are connected in a different order. C 2 H 6 O CHH H CH H

OH OCCHH

H HH H

Ethyl Alcohol Dimethyl Ether

Colorless liquid

mp -117°C bp 78.5°C density 0.789 g/mL (20°C)

IntoxicantColorless gas

mp -139°C bp -25°C density 0.00195 g/mL (20°C)

Refrigerant

Examples: Isomers

• Draw all possible structures having the formulas C 4 H 10 , C 5 H 12 , and C 6 H 14 . C 7 H 16

9 isomers

C 8 H 18

13 isomers

C 9 H 20

35 isomers

C 10 H 22

75 isomers

C 20 H 42

366,319 isomers

C 30
H 62

4,111,846,763 isomers!

C 40
H 82

62,481,801,147,341 isomers!

Chapter 1 Alkanes

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Examples: Isomers

• Which of the following molecules is a structural isomer of acetone? CH 3 CO CH 3

Acetone

H 2 CCHCH 2 OH H 3 CCH 2 CO OHH 2 CCHCO H H 3 CCH 2 CO H 28

Functional Groups

• Organic molecules are often organized by structures called functional groups, which are characteristic arrangement of atoms which define many of the physical and chemical properties of a class of organic compounds.

- The simplest of the functional groups are the hydrocarbons, which include the alkanes, alkenes, alkynes, and aromatic hydrocarbons.

- Many functional groups contain oxygen atoms, such as alcohols, ethers, aldehydes, ketones, carboxylic acids, and esters.

- Some other functional groups contain nitrogen atoms, such as the amines and amides.

• Molecules with the same functional group tend to share similar chemical and physical properties.

Chapter 1 Alkanes

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Table 1.2 Classes and functional groups of organic compounds

ClassFunctional GroupExample of expanded

structural formulaExample of condensed structural formulaIUPAC / Common name

Alkane None H C

H C HHH H CH 3 CH

3ethane

Alkene

ethene (ethylene)CC CH H C HH H 2 CCH 2

Alkyneethyne (acetylene)CC CCHC CHHH

Aromatic

CCCCC C CCCCC CHH HH

HHbenzene

Alcohol C O H CCOHH

HHH H CH 3 CH 2

OH ethyl alcohol

Ether

COC OCCH

HH H CH 3 OCH 3 methoxymethane (dimethyl ether)H H 30
NH

HAmine CHNHH

HH CH3 NH 2 methylamineAldehyde

Ketone

Carboxylic acid

Ester

AmideC

O HCCO H CC O C C O O C O O C O NH H H CC O CH H HHH H C CC O OHH H H CC O OH H HCH H CC O N HHH HCH 3 CHO CH 3 CCH 3 O CH 3 COHO CH 3 COCH 3 O CH 3 CNH 2

Oethanal (acetaldehyde)

2-propanone (acetone)

ethanoic acid (acetic acid) methyl ethanoate (methyl acetate) ethanamide (acetamide)H HH Table 1.2 Classes and functional groups oforganic compounds

ClassFunctional GroupExample of expanded

structural formulaExample of condensed structural formulaIUPAC / Common name

Chapter 1 Alkanes

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32

A Moderately Complex Organic Molecule

C C CCC C C CC CCC C C C CC CC HOC C CC CCC CH H H HH H H HHH H H H HH HH H H H H HHH H H H HHH H H HHHHH H HHHH HHH

Chapter 1 Alkanes

17 33

Expanded Structural Formulas

•In expanded structural formulas (Lewis formulas, Lewis structures), all atoms and bonds are shown:

HCOH H CH HH HCCH H OH H HHCC H CH HH H HCCH H HH H 34

Condensed Structural Formulas

•In condensed structural formulas, only specific bonds are shown; this is useful in reducing the number of C - H bonds that must be drawn.

H 3 CCH 3 CH 3 CH 3 CH 3 CH 3 CH 2 CH CH 3 CH 3 CH 2 CH 3 CH 2 OHOH CH 3 CH 2 CH 3 CH 2 OCH 2 CH 3 OCH 2 CH 3

Chapter 1 Alkanes

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Line Drawings

•In line drawings (line-angle formulas, skeletal structures, stick figures), bonds are represented by lines; everywhere two lines meet or a line begins or ends is a C atom. H's on C's are not shown (except for emphasis); H's on other atoms must be shown.

OH O 36

Drawing Organic Molecules

HCCCCHHH

HHHH H

HExpanded s

tructuralformula (Lewis structure) CH 3 CH 2 CH 2 CH 3

Condensedstructuralformulas

CH 3 CH 2 CH 2 CH 3 CH 3 (CH 2 ) 2 CH 3

Line drawing

=CH 3 = CH 2 = CH = C

Chapter 1 Alkanes

19 37
C C CCC C C CC CCC C C C CC CC HOC C CC CCC CH H H HH H H HHH H H H HH HH H H H H HHH H H H HHH H H HHHHH H HHHH HHH

Cholesterol

Drawing Organic Molecules

Expanded Structural Formula

Drawing Organic Molecules

CHCH 2 CH 2 CH 2 CH(CH 3 ) 2 H 2 C CH C H 2 CCH 2 C C HCH 2

CHCH CHCH

2 C H 2 C C H 2 CH 2 CHCH 3 CH 3 HOCH 3

Condensed Structure

Chapter 1 Alkanes

20 39

Line Drawing

Drawing Organic Molecules

HO 40

Examples: Drawing Organic Molecules

• Draw acceptable condensed structures and line drawings associated with the following expanded structural formulas.

CH H

HCCCCHHHH

HC HH HH C CC C C CH 3 CCH 3 HHH H H HH H H H CHCH HO CH H H

Chapter 1 Alkanes

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41

Examples: Drawing Organic Molecules

• Draw an acceptable expanded structure and linedrawing for the molecule CH 3 CH 2 CH 2 OH. • Draw an acceptable expanded structure and line drawing for the molecule (CH 3 ) 3 CCH 2 CH(CH 3 )CH 3 . 42

Examples: Drawing Organic Molecules

• Draw acceptable expanded structures, condensed structures, and line drawings for the following molecules:

- isopropyl alcohol, CH 3

CH(OH)CH

3 - acetic acid, CH 3 COOH - acetaldehyde, CH 3 CHO - acetone, CH 3 COCH 3

Chapter 1 Alkanes

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43
44

Hydrocarbons

•Hydrocarbons - compounds that contain only carbon and hydrogen. •Saturated Hydrocarbons - contain only carbon-carbon single bonds. •Unsaturated Hydrocarbons - contain carbon-carbon double or triple bonds. CH H HCH H H

Alkanes

CH H CH H CHCHC C CCC CH H H HHH

AlkenesAlkynes

A romatics

Chapter 1 Alkanes

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45

Alkanes

•Alkanesare saturated hydrocarbons - each carbon holds the maximum number of hydrogen atoms). - Alkanes contain only carbon-carbon single bonds. - General formula: C n H 2n+2 (no rings).

• Most chemical reactions require a functional group "handle" to proceed. Since alkanes don't really have functional groups, they aren't very useful in many biologically important processes.

- Since alkanes undergo combustion easily, they are a good source of energy (e.g., gasoline).

- Alkanes also provide the raw materials for the production of many other more complex substances (plastics, etc.).

Some Common Alkanes

•Methane, CH 4

- major component of natural gas (~85%), which is produced by bacterial decomposition of organisms in the absence of oxygen (marsh gas, cow flatulence).

- burns cleanly, so is useful for cooking. - odorless - ethanethiol is added to make natural gas leaks detectable. •Ethane, CH 3 CH 3 (C 2 H 6 ) - a minor component of natural gas (~10%). •Propane, CH 3 CH 2 CH 3 (C 3 H 8 ) - used as an industrial fuel, and in home heating and cooking.

Chapter 1 Alkanes

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47

Some Common Alkanes

•Butane, CH 3 CH 2 CH 2 CH 3 (C 4 H 10 ) - cigarette lighters - Butane is an unbranched (normal)alkane. There is also a branched alkane with the formula C 4 H 10 , having a three-carbon chain with a one-carbon group connected to the middle. - We must give the other isomer a different name: CH 3 CH(CH 3 )CH 3 [or CH 3 CH(CH 3 ) 2 ] is named isobutane (or 2-methylpropane). - Butane and isobutane are structural isomers of each other.

ButaneIsobutaneCH

3 CH 2 CH 2 CH 3 CH 3 CHCH 3 CH 3 48

Conformations of Alkanes

•Conformation - the different arrangements of atoms in space achieved by rotation about single bonds.

• Structures which are related to each other by rotation around a single bond are the same molecule.

CH 3 HHCH 3 HHCH 3 HHH HCH 3 CH 3 HHH CH 3 HCH 3 HH CH 3 HH CH 3 CH 3

HHHHCH

3 HHH H CH 3 CH 3 CH 2 CH 2 CH 3

Chapter 1 Alkanes

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49

Examples: Conformations and Isomers

• Which of the following groups represent structural isomers, and which are simply the same compound?

CH 3 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH 3 CH CH 3 CH 3 CH 3 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 3 CH 3 CH 3 CH 3 CH 2 CH 2 CH 3 CH 3 50

Examples: Conformations and Isomers

• Which of the following groups represent structural isomers, and which are simply the same compound?

CH 3 CHCH 3 CH 3 CH 3 CH CH 3 CH 3 CH 3 CH 2 CH CH 3 CH 3 CH 3 CH CH 3 CH 2 CH 3 CH 3 CCH 3 CH 3 CH 3 CH 3 CH CH 3 CH 2 CH 3 CH 3 CHCH 3 CH 2 CH 3 CH 3 CH CH 2 CH 3 CH 2 CH 3 CH 3 CH 2 CHCH 3 CH 2 CH 3

Chapter 1 Alkanes

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51

Alkane Nomenclature

• Straight-chain alkanes are named by combining a prefixwhich indicates the number of carbon atoms in the chain, and a suffixindicating the functional groupof the molecule.

No. of C's Prefix

1meth-

2eth-

3 prop-

4 but-

5pent-

6hex-

7hept-

8oct-

9 non-

10 dec-FunctionalGroup Suffix

Alkane -ane

Alkene -ene

Alkyne -yne

Alkane Nomenclature

• When alkanes are branched, things get more complex. Remember there are two isomers of C 4 H 10 : • There are three isomers of C 5 H 12 : • There are 75 isomers of C 10 H 22
! • We need a way to name molecules that doesn't require memorizing a huge number of prefixes.

ButaneIsobutaneCH

3 CH 2 CH 2 CH 3 CH 3 CHCH 3 CH 3

PentaneIsopentaneCH

3 CH 2 CH 2 CH 2 CH 3 CH 3 CHCH 2 CH 3 CH 3 CCH 3 CH 3 CH 3 CH 3

Neopentane

Chapter 1 Alkanes

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53

IUPAC System of Chemical Nomenclature

• The system of nomenclature used to name organic compounds was developed by the International Union of Pure and Applied Chemistry (IUPAC).

-A root identifies the longest continuous chain of carbon atoms. -A suffix identifies the main functional group in the molecule.

-A set of prefixes identifies the numbers and positions of the substituents (groups which are attached to the longest chain). (Alkyl groups are substituents which contain a carbon chain.)

Prefix Ending

longest carbon chain functionalclass Root number and identity of attached groups 54

IUPAC Nomenclature of Alkanes

•Step 1.Identify and name the longest continuous chain of C atoms (#C + -anefor alkanes). If there is more than one way to get the same # of C's in the longest chain, use the one that gives more substituents.

CHCH 3 CH 2 CH 3 CH CH 2 CH 3 CH 2 CH 3 CH 3 CH CH 2 CH 3 CH CH 3 CH 3 CH CH 2 CH 2 CH CH 2 CH 2 CH 3 CH 3 CH 2 CH 3 CH 3 CH 3 CH CH 2 CH 2 CH 3 CH CH 3 CH 3 CCH 3 CH 3 CH 3 CH 3

Chapter 1 Alkanes

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55

IUPAC Nomenclature of Alkanes

•Step 2. Number the atomsin the longest chain.

- Number consecutively from the end that will give the lower number to any C to which a group is attached.

- If two or more alkyl groups are attached to the longest chain, use the numbering path that gives the lowest number for the first point of difference.

- If two different alkyl groups are attached at the same distance from either end of the chain, the one that comes first in alphabetical order has the highest priority.

IUPAC Nomenclature of Alkanes

•Step 3.Name the alkyl groups(#C + -yl) and other substituentsconnected to the longest chain. In front of each alkyl group name, put the number of the carbon the group is attached to, separated from the name by a dash (e.g., 2-methyl).

•Step 4.If there is more than one of a particular substituent, combine them into a single word using the appropriate counting prefix(di-, tri-, tetra-, etc.). Include all of the carbon numbers which the groups are attached to, separated by commas (e.g., 2,2,3-trimethyl).

No. of Groups Prefix No. of Groups Prefix

1 - 6hexa-

2di- 7hepta-

3tri- 8octa-

4 tetra- 9 nona-

5 penta- 10 deca-

Chapter 1 Alkanes

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57

IUPAC Nomenclature of Alkanes

•Step 5. Arrange the alkyl groups in front of the parent name in alphabetical order(ignoring counting prefixes, sec-and tert-; iso-isused in alphabetizing). Separate numbers from each other by commas, and numbers from words by dashes.

CHCH 3 CH 2 CH 3 CH CH 2 CH 3 CH 2 CH 3 CH 3 CH CH 2 CH 3 CH CH 3 CH 3 CH CH 2 CH 2 CH CH 2 CH 2 CH 3 CH 3 CH 2 CH 3 CH 3 CH 3 CH CH 2 CH 2 CH 3 CH CH 3 CH 3 CCH 3 CH 3 CH 3 CH 3 58

Examples: Alkane Nomenclature

• Draw structural formulas and give the correct names for all of the possible structural isomers of butane (C

4 H 10 ).

Chapter 1 Alkanes

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59

Examples: Alkane Nomenclature

• Draw structural formulas and give the correct names for all of the possible structural isomers of pentane (C

5 H 12 ). 60

Examples: Alkane Nomenclature

• Draw structural formulas and give the correct names for all of the possible structural isomers of hexane (C

Chapter 1 Alkanes

35
69
70

Cycloalkanes

• Alkanes may also possess cyclicstructures in addition to the straight-and branched-chain acyclicmolecules we have already seen. • General formula: C n H 2n (for one ring) CCC HH HH H H

AcyclicCyclic

PropaneCyclopropane~ 60°

CH 3 CH 2 CH 3 cyclobutane cyclopentanecyclohexanecyclooctane Note that these molecules are notstructural isomers of each other!

Chapter 1 Alkanes

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71

Cycloalkane Nomenclature

• When naming cycloalkanes, the ring is taken to be the longest chain; the prefix cyclo- is added to the norm al root + -suffix. • When mono-substituted cycloalkanes are name d, it is not necessary to specify the position num ber, since all positions in the ring are equivalent. • When more than one substitu ent is located on a ring, the numbering begins at the carbon to which the group is attached which com es first in alphabetical order, and then proceeds in a direction which gives the lowest possible number to the next attached group. 72

Examples: Cycloalkane Nomenclature

• Provide acceptable IUPAC names for the following molecules: CH 3 CH 3 CH 3 Cl CH 3 CH 3

Chapter 1 Alkanes

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73

Examples: Cycloalkane Nomenclature

• Provide acceptable IUPAC names for the following molecules: CH 2 CHCH 3 CH 3 CHCH 3 CH 3 CH 3 CH 3 CH 3 CH 2 CH 3 CH 3 Cl Cl Cl 74

The Shape of Cycloalkanes

• Cyclopropane has bond angles of 60°, which is bent far away from the "normal" 109.5° bond angles of straight-chain alkanes. It is a flat molecule.

• Cyclobutane has bond angles of about 90°; it is also less stable than a "normal" alkane. It is mostly flat, but there is some slight puckering of the ring.

HH HHH H H

H~ 90°

H HHH H

H~ 60°

Chapter 1 Alkanes

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75

The Shape of Cycloalkanes

• Cyclopentane has bond angles of about 108°; it forms a mostly flat but slightly puckered ring. Cyclopentane rings are very common in nature.

• If cyclohexane were flat, the bond angles would be about 120°; but this molecule can adopt a "chair" or "boat" conformation in which the bond angles are 109.5°. Cyclohexane rings are extremely common.

"chair" confo rmation"boat" conformation ~ 108°H H HHH H H H HH 76

Stereoisomers of Cycloalkanes

• The molecules below are different molecules because there is no free rotation around carbon-carbon bonds in cycloalkanes.

• These molecules are stereoisomers - compounds with the same molecular and structural formula but different spatial arrangements of atoms.

• Stereoisomers in which the spatial arrangement is maintain by rings (or double bonds) are called geometric isomersor cis-transisomers.

CH 3 CH 3 CH 3 CH 3 cis-1,2-dimethylcyclopentanetrans-1,2-dimethylcyclopentane

Chapter 1 Alkanes

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77

Examples: Stereoisomers

• State whether each possible pairing of the molecules below are structural isomers, geometric isomers, or the same molecule.

CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 78

Examples: Stereoisomers

• Provide acceptable IUPAC names for the following molecules: Br BrBr Br

Chapter 1 Alkanes

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79
80

Physical Properties of Alkanes

• Since alkanes are composed of relatively nonpolar C - C bonds and C - H bonds, alkanes are nonpolar molecules.

• Because they have only weak attractions for each other, they tend to have lower melting points and boiling points than other organic compounds of comparable molecular weights.

• The straight chain alkanes make up a homologous series in which each members differs from a previous member by having one additional CH

2 group. In a homologous series, the physical properties are closely related and vary in a systematic way.

Chapter 1 Alkanes

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81
82

Physical Properties of Alkanes

• The general rule when judging solubility is "like dissolves like" - polar substances mixes with polar substances, nonpolar with nonpolar, but not polar with nonpolar.

• Alkanes (nonpolar) are insoluble in water(polar), and since they are less dense than water, they float(e.g., oil slicks).

• Alkanes and other substances that do not dissolve in water are often referred to as being hydrophobic("water fearing").

• Liquid alkanes of high molecular weight serve as emollients (skin softeners) to replace oils washed away by bathing or swimming.

- Vaseline is a semisolid mixture of alkanes.

Chapter 1 Alkanes

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83

Alkane Reactions

• Alkanes are the least reactive of all organic compounds. They do not usually react with strong acids or bases, or with most oxidizing or reducing agents.

• They do, however, burn very easily in combustion reactions, releasing a great deal of energy: CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2

O(g) + 212.9 kcal

C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2

O(g) + 488.8 kcal

2C 8 H 18 (g) + 25O 2 (g) 16CO 2 (g) + 18H 2

O(g) +

2448 kcal

Alkane Reactions

• In the absence of enough oxygen for complete conversion to carbon dioxide, some common waste products are generated in the incomplete burning of alkanes:

CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) CH 4 (g) + 3 / 2 O 2 (g) CO(g) + 2H 2 O(g) CH 4 (g) + O 2 (g) C(s) + 2H 2 O(g)

- CO, carbon monoxide, is poisonous, colorless, and odorless. In the exhaust train of most cars, a catalytic converter converts CO to CO

2 .

- Solid elemental carbon produces engine deposits; but this reaction is done to produce lampblack, which is used in some ink pigments.

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85

Alkyl Halides

•Alkyl halides, or haloalkanes, are alkanes in which one or more hydrogen atoms are replaced by halogen atoms (F, Cl, Br, or I).

• Most alkyl halides are not very water-soluble. Alkyl fluorides and chlorides have densities that are higher than those of alkanes, but still less than that of water. Alkyl bromides and iodides are generally more dense than water. Compounds containing more than one halogen are often more dense than water.

• Alkyl halides are named as alkanes with halo-substituents (fluoro-, bromo-, chloro-, and iodo-).

• A number of simple alkyl halides are better known by their common names; for instance, CHCl 3 , trichloromethane, is almost always referred to as "chloroform." 86

Examples: Nomenclature of Alkyl Halides

• Provide acceptable IUPAC names for the following molecules: Cl ClCl CHCH 3 Cl CH 2 CHCH 3 Br CHCH 3 Cl CH 2 CHBr CH 2 CH 2 CH 3 CCl Cl CH 3 Cl CF F CF H H F

Chapter 1 Alkanes

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87

Some Common Alkyl Halides

Dichloromethane

(methylene chloride)

A colorless , mildly toxic

liquid (bp 41°C) more dense than water. It is used as a paint remover and degreaser.

It is also used to decaffeinate

coffee beans; since it has such a low boiling point, the residual solvent can be removed from the beans at fairly low temperatures.C ClCl

HHTrichloromethane (chloroform)

A colorless liquid (bp 60°C); a

very commonly used organic solvent. Chloroform vapor is a anesthetic: James Young

Simpson was the first to use

chloroform as an anesthetic during childbirth in 1846 (presumably, not on himself!), and it was widely used in surgery in the 19th and early 20th centuries. However, since chloroform is carcinogenic, and toxic to the liver, it is not widely used for this purpose anymore.C ClCl

ClHTetrachloromethane

(carbon tetrachloride)

Formerly a common organic

solvent, and was widely used for dry cleaning and spot removal; it has been shown to be toxic and carcinogenic, and contributes to ozone depletion, so it has been replaced by other solvents.C ClCl ClCl 88

Some Common Alkyl Halides

1,1,1-Trichloroethane

Formerly a very commonly used

organic solvent; heavily used in dry cleaning, but it has been replaced by other solvents (such as tetrachloroethylene).C ClCl

CCl HH

HBromochlorodifluoromethane (Halon 1211)

An example of a halon, a haloalkane that has

bromine atoms in addition to chlorine and fluorine atoms. Halons are very stable, and are useful in fire extinguishers, since they do not damage electronic equipment. Their use has largely been phased out under the Montreal Protocols, but they are still used in fire suppression systems aboard some aircraft, since no completely satisfactory and safe al ternatives havebeendiscovered.C FF ClBr

Chapter 1 Alkanes

45
89

Chlorofluorocarbons (CFCs)

Dichlorodifluoromethane (Freon-12)

An example of the chlorofluorocarbons (CFCs, or

freons), developed in the 1920s; they are relatively nontoxic, very unreactive, and boil at low temperatures, and were thus ideal for use as refrigerants; they were also widely used as aerosol propellants and as foaming agents. Unfortunately, they persist in the environment for a long time (up to a century), and make their way into the upper atmosphere, where they are split by high energy light from the Sun, releasing chlorine atoms. These Cl atoms destroy ozone in the stratospheric ozone layer that shields us from much of the Sun's UV radiation. (F. Sherwood Rowland, Mario J. Molina, Paul Crutzen, Nobel Prize in Chemistry,

1995) In 1987, a treaty called the Montreal Protocol on

Substances that Deplete the Ozone Layer was signed, which cut back on the production and use of CFCs; in 1990, in response to the alarming increase in the size of the "ozone hole" over the South Pole, the agreement was extended to become a ban on the use of CFCs starting in 2000.C ClCl

FFChlorodifluoromethane (Freon-22)

An example of a hydrochlorofluorocarbon

(HCFC), developed as alternatives to the

CFCs. The HCFCs are not fully halogenated,

and are less stable than the CFCs, and degrade before they reach the upper atmosphere.C HCl FF

1,1,1,2-Tetrafluoroethane (Freon-134a)

A hydrofluorocarbon (HFC), another

group of CFC-alternatives that are not damaging to the ozone layer. Freon-134a is now widely used in the air conditioning systems of automobiles in place of F reon-12.C FF CFFH H 90

Petroleum

•Petroleumis a mixture of hydrocarbons formed over millions of years, primarily from the decay of microscopic ocean-dwelling plants and animals. The resulting crude oil collects in underground pockets in sedimentary rock.

• Petroleum is separated into different fractions by fractional distillation.

• Most petroleum products are burned as fuel, but about 2% is used to synthesize other organic compounds. (That's still a lot!)

• Over half of all synthetic industrial organics, including dyes, drugs, plastics, fibers, detergents, insecticides, etc., are made from petroleum sources

Chapter 1 Alkanes

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91
Fraction Boiling RangeMolecular-size range Typical uses

Gas -164-30°C C

1 -C 4

Heating, cooking

Gasoline 30-200°C C

5 -C 12

Motor fuel

Kerosene 175-275°C C

12 -C 16

Fuel for stoves; diesel and jet engines

Heating oil Up to 375°C C

15 -C 18

Furnace oil

Lubricating oils350°C-up C

16 -C 20

Lubrication, mineral oil

Greases Semisolid C

18 -upLubrication, petroleum jelly

Paraffin (wax)Melts 52-57°C C

20 -up Candles, toiletries

Pitch and tar

Residue in boilerHigh Roofing, asphalt paving

Petroleum Fractions

Carbon, in fact, is a singular element: it is the only element that can bind itself in long stable chains without a great expense of energy, and for life on earth (the only one we know so far) precisely long chains are required. Therefore carbon is the key element of living substance: but its promotion, its entry into the living world, is not easy and must follow an obligatory, intricate path . . . If the elaboration of carbon were not a common daily occurrence, on the scale of billions of tons a week, wherever the green of a leaf appears, it would by full right deserve to be called a miracle.

Primo Levi, "Carbon" in

The Periodic Table (1975)

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